Many authors now agree there is a significant possibility of abrupt discontinuities in the earth's climate, e.g. by global warming, through rising atmospheric greenhouse gas levels which in turn may cause significant positive feedback, such as release of methane from melting arctic tundra or loss of rainforest through changed precipitation patterns. Such discontinuities might lead to a serious impact on ocean levels, and agriculture.
However, emissions of greenhouse gases such as CO2, are intimately connected to the economies of the world. An abrupt reduction in the levels of these gases over effective timescales may be difficult to achieve.
Various methods of reducing the levels of solar radiation incident upon the earth have been proposed, for example, space minors, cloud seeding and stratospheric sulphate aerosols. The effects of significant stratospheric sulphate aerosol injection have been demonstrated to reduce mean global temperatures by around half a degree centigrade over a period of two years with the eruption of Mount Pinatubo in the Philippines in 1991.
Reducing incident solar radiation to counteract the trapping of heat by rising greenhouse gas emissions leaves ocean acidities rising due to increased levels of absorbed carbon dioxide. However it may reduce rising temperatures generating more greenhouse gas emissions through the melting of arctic permafrost exacerbating the ocean acidity problem.
The particular method of stratospheric sulphate aerosols suffers from a number of drawbacks: the choice of particle diameter is determined by natural processes in the stratosphere if precursor materials such as Sulphur Dioxide or Hydrogen Sulphide are used, and the generation of stable aerosols with defined characteristics presents many difficulties. Furthermore their effect on stratospheric chemistry, particularly the ozone concentration may cause concern. In addition the possibility of generating local ‘shadows’ does not arise since it takes a significant time (days at least) after injection of Sulphur Dioxide for Sulphuric Acid or Ammonium Sulphate particles to be generated by the natural processes (hydration and reaction with UV atmospheric Nitrous Oxides), by which time any high local concentration will be well dispersed by even moderate stratospheric winds.
There is therefore a need for more effective solutions to the problem of global warming with less alarming side effects.